When you install SharePoint Server 2013 the Distributed Cache service is configured to use 10% of the RAM on the server. Half of the 10% is used for caching and the other half is used for memory management. If you add RAM to the server you need to manually re-configure the Distributed Cache service to make use of the extra RAM.

To check the current RAM allocation run the command below from an elevated SharePoint 2013 Management Shell, replacing the server name as necessary.